A smoke warning lit up on a passenger plane as it flew over Cumbria, prompting actions that “significantly degraded” the aircraft’s “operational capability”, a report reveals today.

The warning light lit up as the aircraft flew south west of Carlisle in west Cumbria.

The pilots of the Edinburgh-bound Dash 8 aircraft, with 47 passengers on board, assumed it was valid and carried out checklist actions designed to tackle fire or smoke.

These caused the aircraft to begin to depressurise, the commander had to fly manually and the co-pilot’s flight displays were blank.

But the warning turned out to be false – and was probably caused by a short circuit.

The warning light had lit up as the aircraft flew over an unspecified location, about 25 miles south west of Carlisle in west Cumbria. The plane was operated by airline Flybe and was travelling to Scotland from Newquay Airport in Cornwall.

The alert happened mid-morning on July 21 last year, a report from the Air Accidents Investigation Branch (AAIB) said. The co-pilot had asked the senior cabin crew member to look for smoke and she reported back that she could not smell any smoke.

The captain told her that they “had to fight the fire” and that they would be making an emergency descent into Edinburgh, the report added.

The AAIB said that the captain had told the co-pilot that no smoke had been seen but they agreed that the situation would have to be treated “as real”.

When the plane landed safely at Edinburgh, passengers were evacuated.

The AAIB said: “The crew were presented with a smoke warning but there were no corroborating signs of smoke or fumes.

“The pilots were not prepared to proceed on the basis that the warning was spurious and assumed that it was valid.” The report went on: “Subsequently, the continuing absence of corroborating evidence did not alter this assumption.

“Having made their assumption the pilots carried out checklist actions designed to remove smoke from the aircraft.